Taming your machine with a mouse has long been a dream of the ATARI 8-bitters.
The mouse had its origins around the 1960's, but did see much use until the
1980's when the Mac,ST and Amiga were unveiled. The mouse was also used by the
engineering world as an input device for graphics workstations in the late
70's. The Track-ball is a close relative to the Mouse. A track-ball is
basically the same device flipped over allowing the hand position the ball
directly.

The Old ATARI did have some foresight in developing input devices for the 8-bit
machines. The TrackBall is one of them. The Track-ball allows smooth tracking
of 2 dimensional motion and its associated velocity. Atari Basic is too slow
so it can not be read with atari basic. Faster languages such ACTION and
Assembly can read the velocity vectors directly from the joystick registers.
(mention something about which bits represent the direction and speed.)
One thing the Atari Trak Ball lacks is a separate button that can function as
the left mouse button. Since the mouse and the Track-ball are virtually the
same device it should be possible to read and ST mouse using the Trak-ball read
code. The 8-bits can read the ST left mouse if a pull-up resistor is added to
pin 6.

I have included 3 programs that demostrate the trak-balls ability to read
direction and velocity. One program is written in basic with a short assemble
used to read the T-ball input vector. The other 2 programs are written in
ACTION!